It is known to investigate samples with beams of electromagnetism using ellipsometer systems. It is also known that unaccounted for changes in the characteristics of a beam of electromagnetic radiation, (ie. noise content), used to investigate a sample can adversely affect the results achievable by practice of ellipsometry.
As it is a particularly relevant area for application of the present invention, it is noted that a benefit of ellipsometry is that data produced thereby consists of a phase angle between, and a ratio of the intensities of, orthogonal components in a polarized beam. As changes in beam intensity generally affect both orthogonal components of the polarized beam, it is generally believed that forming a ratio of the intensities of the orthogonal components cancels out random variations in a beam of electromagnetic radiation provided by the affected source. However, it can occur that, for instance, data detectors operate differently when receiving different intensity signals, thus it can, even in ellipsometry, be of benefit to compensate for changes in intensity of a beam of electromagnetic radiation which interacts with a sample prior to it entering into a data detector. Further, it can occur that source provides electromagnetic radiation having noise with a frequency component which is substantially the same as a frequency at which a beam of electromagnetic radiation is modulated by an ellipsometer. Without some approach to separating the effects thereof from modulation desirably intentionally imposed on beam intensity, the results of analysis of a sample will be in error.
It is also noted that changes in the intensity of a beam of electromagnetic radiation entering a detector can result from not only from changes in output intensity of a source, but can result from said beam being reflected from a sample which is, for instance, rotated during data collection to allow investigation of more than one spot thereon, or has material deposited thereupon during sample investigation which changes the characteristics of a single investigated spot. This effect can be appreciated by realizing that If the sample does not have a truely flat surface and/or is not perfectly aligned and rotated about an axis which is oriented truely normally to an investigated surface thereof, differing amounts of incident electromagnetic radiation will reflectively enter a data detector therefrom, depending on at what point in a sample rotation cycle said sample is investigated. And, depositing material onto a sample also changes the amount of incident electromagnetic radiation will reflectively enter a data detector. Similar effects can result when a sample is caused to undergo a linear motion, such as when a ribbon sample is unwound from one spool and wound onto another, with the beam of electromagnetic radiation being directed to interact with said ribbon sample at a point between said spools. It is noted that identified noise can then be random and/or periodic.
A relevant identified U.S. Pat. No. 7,342,661 to Ebert et al. Said 661 patent describes a normalization procedure for data in a rotating element ellipsometer system involving applying D.C. components from half cycles to, respectively, normalize data in said half cycles. U.S. Pat. Nos. 7,151,605 and 6,831,740 to Herzinger et al., describe methodology for efficiently providing good data of improved precision over a range of wavelengths.
Other relevant art includes dual channel ellipsometers, such as described in U.S. Pat. Nos. 7,564,552; 7,433,037; 7,359,052; 7,349,079; 7,067,819; 6,721,052 and 5,548,404. In it disclosed that a difference between such dual channel systems and the present invention is that the former typically normalizes an overall signal average by a reference detector signal, while the present invention teaches that every data point is separately normalized. Normalizing each data point separately can lead to, for instance, determining Fourier coefficients which are extracted form an ellipsometer signal with improved precision and accuracy. This, in turn, leads to improved accuracy in ellipsometric data.
A system which comprises means for compensating undesired “noise” caused by changes in electromagnetic beam intensity entering a detector, (such as those resulting from random source output variation and those resulting from typically periodic sample movement), in ellipsometers, polarimeters or the like, and the methodology of its application, would therefore provide utility. The present invention provides methods for improving the precision and accuracy of ellipsometric data by normalizing a “true data” containing signal to unintended variations in beam intensity which are not the result of intended applied modulation in the ellipsometer system, such as are caused by variations in beam source intensity and/or by a moving substrate.